Step 1: In a large bowl, using an electric mixer, cream together the softened unsalted butter, light brown sugar, and granulated sugar on medium speed until light and fluffy, about 2-3 minutes. Scrape down the sides of the bowl as needed.
Step 2: Beat in the large eggs one at a time, mixing well after each addition. Stir in the vanilla extract until just combined.
Step 3: In a separate medium bowl, whisk together the all-purpose flour, baking soda, ground cinnamon, and salt, ensuring they are well combined.
Step 4: Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Be careful not to overmix the dough. A few streaks of flour are acceptable.
Step 5: Fold in the old-fashioned rolled oats until evenly distributed. If adding optional mix-ins (like raisins or chocolate chips), add them now.
Step 6: Cover the bowl with plastic wrap and refrigerate the dough for at least 30 minutes, or up to 24 hours. This chilling step is crucial for texture and flavor.
Step 7: Preheat your oven to 375°F (190°C). Line baking sheets with parchment paper or silicone baking mats.
Step 8: Drop rounded tablespoons of dough onto the prepared baking sheets, leaving about 2 inches between each cookie.
Step 9: Bake for 10-12 minutes, or until the edges are golden brown and the centers are still slightly soft. They will continue to set as they cool. Do not overbake.
Step 10: Remove the baking sheets from the oven and let the oatmeal cookies cool on the sheets for 5 minutes before transferring them to a wire rack to cool completely.